It is often important and sometimes critical that a back up power source be available in case a primary power source is either unavailable or degrades until it is not suitable for powering a load. A hospital or a large computer center may, for example, have access to two separate sources of alternating current power for operating some or all of the equipment.
A prior art switched power source is disclosed in U.S. Pat. No. 5,138,184 to Keefe. This patent discloses a solid state, static power supply control system for transferring a load from a preferred source of AC power to an alternate source of AC power.
Cyberex Inc., assignee of the present invention, is one of a number of suppliers of solid state switches. These switches utilize pairs of gate activated silicon controlled rectifiers or SCRs that are connected in parallel. Each pair of SCRs conducts current from a single pole of alternating current to a load. One SCR conducts current in one direction and a second, oppositely connected SCR conducts current in a second direction as the AC source switches polarity. When both SCRs are gated into conduction, first one and then the other SCR provides a low resistance path for alternating current power as current flow alternates back and forth during the AC power cycle.
A transfer from a faulty power source to an alternate power source requires the active or conductive SCRs be de-activated and a second set of SCRs pairs be activated to couple an alternate power source to the load. Such a transfer should be accomplished with a minimum disruption of current flow to the load and also should be accomplished with no current transfer between the two power sources.
Present gate control algorithms generally use a technique that suffers certain deficiencies. Upon initiation of a transfer command, a first set of SCRs that are carrying current are ungated. When current stops flowing through this first set of SCRs, a second set of SCRs through which current is to begin passing are gated on. For multiple pole switches, either individual poles are treated separately, or entire banks of poles associated with the ports are first ungated and then a new bank gated on.
Prior art methods guarantee current disruption during the switching process. Current disruption for one half of a 60 hertz power signal is not important for most applications but can be critical in data monitoring and processing operations. Present switching methods result in particularly severe disruption of current when three phase power sources are not exactly aligned in phase, or do not share a common neutral connection.
While a preferred use of the invention is in switching between alternate power sources, the invention has use in switching between alternate loads as well. In this alternate application a single power source is connected to two or more alternate loads.